detection of tar brown carbon with the single particle ... · the sp2 is normally used to report...

Detection of tar brown carbon with the single particle sootphotometer (SP2)Joel C. Corbin 1 and Martin Gysel-Beer 2

1Metrology Research Centre, 1200 Montreal Road, National Research Council Canada, Ottawa, ON K1A 0R6, Canada2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

Correspondence: Joel Corbin ([email protected])

Abstract.

We investigate the possibility that the refractory, infrared-light-absorbing carbon particulate material known as “tar balls”

or tar brown carbon (tar brC) generates a unique signal in the scattering and incandescent detectors of the single particle soot

photometer (SP2). As recent studies have defined tar brC in different ways, we begin by reviewing the literature and proposing

a material-based definition of tar. We then show that tar brC results in unique SP2 signals due to a combination of complete5

or partial evaporation, with no or very little incandescence. Approximately 70% of tar particles incandesced. At the time of

incandescence the ratio of light scattering to incandescence from these particles was up to twofold greater than from soot

black carbon (BC). In our sample, where the mass of tar was threefold greater than the mass of soot, this led to a bias of < 5%

in SP2-measured soot mass, which is negligible relative to calibration uncertainties. The enhanced light scattering of tar is

interpreted as due to its being more amorphous and less graphitic than soot BC. The fraction of the tar particle which does10

incandesce was likely formed by thermal annealing during laser heating.

These results indicate that laser-induced incandescence, as implemented in the SP2, is the only BC measurement technique

which can quantify soot BC concentrations separately from tar, while also potentially providing real-time evidence for the

presence of tar. In contrast, BC measurement techniques based on thermal–optical (“EC”) and absorption (“eBC”) measurements

cannot provide such distinctions. The optical properties of our tar particles indicate a material similarity to the tar particles15

previously reported in the literature. However, more- and less-graphitized tar samples have also been reported, which may show

stronger and weaker SP2 responses, respectively.

1 Introduction

Atmospheric light-absorbing carbon (LAC) in particulate matter (PM) plays a substantial role in the radiative balance of the earth

both directly and by influencing cloud properties (Boucher et al., 2013). While soot black carbon (soot BC) is the best-recognized20

form of LAC (Bond et al., 2013), increasing attention has recently been paid to the so-called “brown carbon” (Kirchstetter

et al., 2004; Laskin et al., 2015) and “tar balls” (Pósfai et al., 2004; Hand et al., 2005; Niemi et al., 2006; Semeniuk et al.,

2006; Tivanski et al., 2007; Alexander et al., 2008; Vernooij et al., 2009; Chakrabarty et al., 2010; Adachi and Buseck, 2011;

China et al., 2013; Zhu et al., 2013; Tóth et al., 2014; Hoffer et al., 2016a, b; Sedlacek III et al., 2018; Corbin et al., 2019)

1

https://doi.org/10.5194/acp-2019-568Preprint. Discussion started: 11 July 2019c© Author(s) 2019. CC BY 4.0 License.

which possess substantially different physical properties than BC. The term “brown carbon” is canonically used to refer to the25

collection of substantially light-absorbing organic molecules found in PM, while the term “tar balls” refers to the insoluble

amorphous-carbon spheres which may be produced by the pyrolysis of high-molecular-weight fuels such as biomass (Tóth et al.,

2014) or heavy fuel oil (Corbin et al., 2019). Here we will refer to these two sub-types of brown carbon as “soluble brown

carbon” (soluble brC) and “tar brC” following Corbin et al. (2019). Both forms of brC may comprise a large fraction of the light

absorption of atmospherically-relevant aerosols such as wildfire smoke (Lack et al., 2012; China et al., 2013; Liu et al., 2015)30

and marine-engine exhaust (Corbin et al., 2018b, 2019).

The distinction between soluble brC, tar, and soot BC is important as it may result in unique environmental fates and impacts

of these distinct types of LAC, due to their unique chemical and morphological properties (Corbin et al., 2019). Similarly,

“BC” instruments designed to measure soot BC based on one of its defining properties – insolubility in water and organic

solvents; refractoriness up to ∼ 4000 K; a structure consisting primarily of sp2-bonded graphene-like carbon; and a morphology35

of aggregated monomers of diameter 10–80 nm (Bond et al., 2013; Petzold et al., 2013) – may be cross-sensitive to soluble

brC or tar. Cross-sensitivities to soluble brC are generally limited to the BC instruments measuring light absorption at shorter

visible wavelengths of 300–500 nm, which normally report their measurements as equivalent BC (eBC, Petzold et al. 2013).

Cross-sensitivities to tar brC are more problematic, and include both eBC measured at visible and near-infrared wavelengths of

300-1000 nm, as well as and thermal–optically defined elemental carbon (EC) (Corbin et al., 2019).40

The fact that tar brC may absorb substantially at wavelengths of 1000 nm implies a further potential cross-sensitivity of

instruments such as the single-particle soot photometer (SP2), which relies upon a continuous-wave 1064 nm Nd:YAG laser

to heat particles to incandescence. The SP2 is normally used to report single-particle rBC mass concentrations by calibrating

incandescence signals with reference to BC particles of known mass and composition. Therefore, any cross-sensitivities of SP2

rBC to tar brC would require that tar be refractory enough to reach the >3000 K attained by soot BC.45

However, an SP2 cross sensitivity to tar brC may also occur indirectly, if a tar brC sample is capable of undergoing complete

or partial annealing to rBC during heating by the SP2 laser. This would result in an incandescence signal from rBC which did

not exist prior to the measurement. A recent study by Sedlacek et al. (2018) suggested that this effect may lead to apparent rBC

signals of up to 9% of the total particle mass, for laboratory-generated tar brC. Their tar brC measurements could not distinguish

whether this signal was due to the formation of rBC during tar-brC formation versus in-SP2 annealing. Sedlacek et al. (2018)50

also performed experiments using nigrosin (a polyaniline-based dye) to demonstrate that laser-induced annealing may contribute

45% of the incandescence signal expected for pre-existing rBC. This percentage decreased with increasing laser power, to

25%, demonstrating that evaporation may occur more rapidly than annealing under appropriate conditions (including high laser

power density and low degree of graphitization in the starting material). It is interesting to note that Moteki and Kondo (2008)

also measured nigrosin in an SP2 but observed zero laser-induced annealing. This may reflect variability in the experimental55

procedures or in the composition of the nigrosin.

Corbin et al. (2019) reported that the apparent rBC mass of tar particles (that is, the magnitude of any cross-sensitivities)

produced by a marine engine operated on heavy fuel oil was negligible. Their inference was based on the fact that thermal–optical

EC (IMPROVE-A protocol) remained high while rBC signals fell to zero, under conditions where the aerosol AAE (the negative

2


slope of a log-log plot of absorption against wavelength) was ∼ 2, corresponding to low engine loads. They also found a60

negligible response of the Soot-Particle Aerosol Mass Spectrometer (SP-AMS; Onasch et al. 2011), which also relies on the

absorption of a 1064 nm laser, to these tar particles. Note that any comparison of the SP2 and SP-AMS sensitivities should be

made with caution, as particles experience lower pressure, shorter beam exposure times, and different laser-power densities in the

latter instrument (laser power densities are unmeasured and may vary between instruments). The lack of SP2 cross-sensitivity to

tar brC in Corbin et al. (2019) is likely related to the fact that tar brC is less refractory than soot BC, and therefore not capable of65

reaching incandescent temperatures prior to vapourization. Another potential factor, that the mass-specific absorption efficiency

(MAE or “MAC”) of the tar brC was 23-fold smaller than that of soot BC at 950 nm wavelength, (although it was only 5-fold

smaller at 660 nm and 2-fold smaller at 370 nm). The MAE is fundamentally related to refractoriness, because both MAE and

refractoriness will increase with increasing degree of graphitization of the tar (Corbin et al., 2019), as will be discussed further

later.70

A negligible incandescence signal in the SP2 does not mean that the SP2 is incapable of detecting tar brC, because the

SP2 does not only measure time-resolved incandescence signals but also time-resolved scattering signals. Previous work has

exploited the time-resolved SP2 scattering signals, particularly relative to the time of incandescence, to determine apparent rBC

coating thicknesses (Gao et al., 2007; Laborde et al., 2012a) and to differentiate core-shell rBC particles from “attached” rBC

particles (BC particles coagulated with, but not engulfed by, non-BC particles, Moteki et al. 2014).75

From this basis, the present manuscript explores the possibility that a detailed analysis of the time-resolved signals allows

the detection of tar brC particles by SP2 in terms of their (predicted) anomalous scattering signals. We also seek to quantify

the potential interference of tar on SP2 incandescence. We use data from Corbin et al. (2019) for our analysis, contrasting a

tar-containing sample with a tar-free sample from the same engine. The manuscript is structured as follows. In Section 2 we

review previous work on the properties and definition of tar brC. In Section 3, we give a technical discussion of the SP2, present80

the test data set used below, and describe the analysis techniques used in this study. In Section 4, we present the results of this

study.

2 Definition and properties of tar brC

2.1 Review of properties

The particles referred to as “tar balls” or “tar brC” in the literature generally have consistent properties. Physically, tar brC exists85

as spheres of solid, amorphous-carbon-like material, and may exist as isolated spheres (Pósfai et al., 2004) or as aggregates

thereof (Girotto et al., 2018). The isolated or aggregated tar particles are of diameters 100–300 nm, approximately one order

of magnitude larger than the corresponding monomer diameter in soot aggregates, 10–80 nm (Corbin et al., 2019). Unlike the

soluble organic molecules (soluble brC) typically addressed by brC studies (Laskin et al., 2015; Moschos et al., 2018), tar brC is

insoluble and vapourizes at about 1000K (Corbin et al., 2019).90

Tar brC consists primarily of carbon but contains impurities of hydrogen and oxygen (Tóth et al., 2018). Tar brC emitted

from biomass burning may contain impurities of K, Cl, Si, and S (Pósfai et al., 2004). Atmospheric tar brC have also been

3


reported which contained impurities of S and Si, but not K (Alexander et al., 2008; Zhu et al., 2013), which has been proposed

as indicating an origin of heavy-fuel-oil combustion (Corbin et al., 2019).

In terms of their optical properties, tar particles may be considered a subset of brC, as their imaginary refractive index95

decreases with increasing wavelength (Alexander et al., 2008; Corbin et al., 2019), which results in a brown appearance at

appropriate concentrations (Liu et al., 2016). However, light absorption by tar remains significant even in in the near-infrared

(880 nm) (Alexander et al., 2008; Corbin et al., 2019). This light absorption has been described by the Tauc band-gap model,

which predicts a slow tailing off of absorption with increasing wavelength (Corbin et al., 2019).

In terms of its molecular structure, tar is best considered as a subclass of the solid carbon materials which lie along a100

continuum of graphitization rather than a complex mixture of discrete molecules (like soluble brC) (Corbin et al., 2019). The

carbon in tar is much less graphitized than the carbon in soot BC; that is, tar has a higher sp3/sp2 bonding ratio than BC (Tóth

et al., 2018; Corbin et al., 2019).

Here we emphasize that a “continuum of graphitization” does not mean that brC and BC are formed by a simple one-

dimensional process of graphitization, and does not mean that tar brC is incompletely-formed soot BC. Studies such as Saleh105

et al. (2018) should not be misinterpreted as suggesting this. First, kinetic limitations to the graphitization process mean that

movement along the “continuum” may be limited for a given starting material (Corbin et al., 2019). Second, there is some

evidence for tar formation during atmospheric processing (Pósfai et al., 2004; Sedlacek III et al., 2018), and not only during

thermal annealing (Tóth et al., 2014; Corbin et al., 2019). Third, the small diameters of soot-BC monomers fundamentally

influence the molecular structure (Parent et al., 2016) and (consequently as well as independently) the optical properties of110

soot-BC aggregates (Liu and Mishchenko, 2005). The fundamental differences in molecular structure and morphology of tar

compared with soot-BC reflect fundamentally different formation pathways; the two LAC types are not simply points along a

one-dimensional continuum (Corbin et al., 2019).

The formation of tar brC has been proposed to proceed through the pyrolysis of high-molecular-weight fuels including biomass

(Tóth et al., 2014) and heavy fuel oil (Corbin et al., 2019), or by the atmospheric processing of primary biomass-burning particles115

(Pósfai et al., 2004; Sedlacek III et al., 2018). Whereas the pyrolysis hypothesis has been directly demonstrated by laboratory

studies for both biomass tar (Tóth et al., 2014) and heavy fuel oil tar (Jiang et al., 2019), the atmospheric-processing hypothesis

is based on less direct evidence from two aircraft-based studies. In the first study, Pósfai et al. (2004) used electron microscopy

to observe tar ball number fractions increasing from a negligible amount to 85% in a smouldering fire over Mozambique after

about 1h of atmospheric processing. It must also be mentioned that Pósfai et al. (2004) also observed a tar ball number fraction120

of 18% for a separate unprocessed plume over South Africa. In the second study, Sedlacek III et al. (2018) also concluded that

tar ball number and mass fractions increasing with plume age. They quantified tar ball number fractions by electron microscopy,

and tar ball mass fractions under the assumption that their tar balls flash-vapourized at 873 K in an aerosol mass spectrometer

(which may not be true for processed tar balls, and which was not observed for other samples (Corbin et al., 2019)). The

chemical mechanism by which photochemical aging may produce amorphous-carbon spheres material is unclear, and should be125

investigated in future work.

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2.2 Definition

As stated above, the term “tar balls” refers to the insoluble amorphous-carbon spheres which may be produced from the pyrolysis

of high-molecular-weight fuels such as biomass (Tóth et al., 2014) or heavy fuel oil (Corbin et al., 2019), and also by atmospheric

processing (Sedlacek III et al., 2018). In this manuscript, we have not measured particle morphology, so we use the term “tar130

brC” to refer to this material, following Corbin et al. (2019). Other literature definitions are as follows.

The term “tar balls” was introduced by Pósfai et al. (2004) to describe the spherical, amorphous-carbon particles found in

biomass burning smoke which were stable under the electron beam of a transmission electron microscope. This definition is

inconvenient for studies which have not used electron microscopy, which cannot assert stability under an electron beam and may

not be able to assert sphericity. In addition, later work by Girotto et al. (2018) showed that “tar ball aggregates” may also exist.135

We therefore prefer the chemically-based definition given above.

Recently, Sedlacek III et al. (2018) described tar balls as processed primary particles, based on their observations of tar balls

appearing to form during atmospheric processing. These observations are corroborated by those of Pósfai et al. (2004), who also

measured more tar balls in more-processed wildfire plumes. However, we propose that this definition would be more useful and

less ambiguous if inverted. Rather than restricting the term tar balls to processed primary particles (a definition which relies on140

knowledge of particle history), we allow that atmospheric processing may form tar balls. We then define tar based on its material

properties as given above.

This inclusive, material-based definition avoids the unnecessary confusion of requiring a separate name for the tar-like

particles measured in unprocessed wildfire plumes (Pósfai et al., 2004; Semeniuk et al., 2006; Adachi and Buseck, 2011; China

et al., 2013); in laboratory biomass smoke (Vernooij et al., 2009; Chakrabarty et al., 2010); in fresh marine-engine exhaust145

(Corbin et al., 2019; Jiang et al., 2019); or in atmospheric air masses of unmeasured or unreported photochemical age (Niemi

et al., 2006; Hand et al., 2005; Tivanski et al., 2007; Alexander et al., 2008; Zhu et al., 2013). As all of these particles possess

similar material properties, a single name is most appropriate for them. Minor differences such as the formation of oxygenated

interfaces due to atmospheric processing (Tivanski et al., 2007) are best considered as forming subcategories within the category

of tar.150

3 Methods

3.1 Technical description of the Single Particle Soot Photometer (SP2)

The single particle soot photometer (SP2; Droplet Measurement Technologies, CO, USA) is designed to quantify the mass of BC

in single particles by laser-induced incandescence (LII, Stephens et al., 2003; Schwarz et al., 2006). Particles exiting a nozzle at

near-atmospheric pressure are guided by a sheath flow through into a continuous-wave, intracavity, 1064 nm Nd:YAG laser over155

the course of roughly 20µs. If particles are heated to incandescent temperatures by this laser, the resulting incandescent light

I(t) is measured by broadband (350–880 nm) and narrowband (350–550 nm) detectors; the broadband signals are normally used

due to their greater sensitivity. Elastic scattering of the laser radiation S(t) is measured by a second detector. A third detector

5


also measures light scattering, but using a four-element avalanche photodiode with the polarity of two elements reversed, such

that the measured signal crosses through zero when a particle reaches a specific physical location relative to the detector. This160

so-called “split detector” allows the absolute location of a particle in the laser beam to be unambiguously identified, by using the

scattering signals of non-evaporating particles as a measurement of the beam profile (Gao et al., 2007; Laborde et al., 2012a). In

the SP2 used during this study, data were acquired from all detectors every 0.4µs. All data in this study were analyzed using the

PSI SP2 Toolkit, version 4.114, to which the novel features discussed below were added.

The SP2 scattering detector was calibrated by reference to polystyrene latex sphere standards of diameter 269 nm. The165

incandescence detectors of the SP2 were calibrated using mass-selected rBC particles with mass-specific incandescence

responses similar to diesel-engine rBC (Alfa Aeser Inc., FS, Lot #FS12S011). As LII signals are influenced by the material

properties of the rBC (Laborde et al., 2012b; Michelsen et al., 2015), it should be kept in mind that the mass calibration for

different materials (such as the rBC in soot or tar) may be different.

The SP2 detects rBC cores with mass (or volume-equivalent size, considering a void-free material density of 1800kg m3)170

from ∼ 0.7 fg (∼ 80 nm) to ∼ 200 fg (∼ 600 nm). Smaller rBC particles can also be detected, although with reduced counting

efficiency (Laborde et al., 2012c). For canonical soot BC, the integrated mass obtained by SP2 has been validated as accurate

by multiple independent studies over more than a decade (Slowik et al., 2007; Kondo et al., 2011; Laborde et al., 2012c). A

substantial amount of work has also employed the SP2 to investigate the internal mixing or “coating” of rBC with volatile

materials (Liu et al., 2017, and references therein). Further details of the SP2 analysis of the present data sets are presented in175

Corbin et al. (2018b).

At least two conditions must be met for particles to incandesce in the SP2. First, they must experience a substantial net

heat input from the 1064 nm laser (over a period of 20µs, due to the particle velocity through the laser). This corresponds

to a minimum required MAE, so that the heating rate exceeds conductive, evaporative, and other cooling rates (Michelsen

et al., 2015; Bambha and Michelsen, 2015). In the SP2, the major cooling mechanism is conductive heat transfer (Bambha and180

Michelsen, 2015), which has been reported as limiting its ability to detect spark-generated carbon nanoparticles (Gysel et al.,

2012). The heating rate will depend most strongly on the material properties of the particle; these properties may themselves be

influenced by the heating process if annealing occurs (Michelsen et al., 2015; Sedlacek et al., 2018). Second, particles must be

refractory up to ∼ 3000 K, so that the corresponding incandescence is detectable (Schwarz et al., 2006). Particles types which

meet these conditions include canonical soot BC (Schwarz et al., 2006) and metal-containing particles such as dust (Moteki185

et al., 2017) and volcanic ash (M. Gysel, unpublished data). Particle types which do not meet these conditions include canonical

non-absorbing materials and tar brC. Non-absorbing materials (such as volatile organics, sulfates, and nitrates) will not absorb

the SP2 heating laser and are not refractory. Tar brC may absorb the SP2 heating laser, but is generally only refractory to about

1000 K (Corbin et al., 2019). Other forms of brC neither absorb substantially above ∼ 500nm (Kirchstetter et al., 2004; Laskin

et al., 2015; Moschos et al., 2018) nor are refractory (Lack et al., 2012).190

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3.2 Test data set

The test data set used here is a subset of the experiments described in previous publications (Corbin et al., 2018b, a, 2019) and

summarized in the introduction. The subset of the data set used here corresponds to a single marine engine operated on the

same heavy fuel oil (HFO) at the same engine load, but with the engine tuning parameters varied such that either a pure-BC or a

tar-rich aerosol resulted (Corbin et al., 2019). We define the relative importance of BC or tar in the samples according to the195

Absorption Ångström Exponent (AAE) calculated from the wavelength pair 370,950 nm. This AAE(370,950) was close to 1.0

for the pure-BC case and ∼ 2.0 for the tar-rich case. The tar-rich case did not correspond to a pure-tar aerosol, but rather one in

which the rBC/EC mass ratio was 0.18, and in which over half of the total light absorption at 950 nm was due to tar and not soot

BC. The pure-BC case corresponded to an rBC/EC ratio of 0.97.

In the analysis herein, the pure-BC case is used as a control case while the tar-rich case is used to identify features unique to200

tar particles. The two cases were measured on the same day using the same sampling configuration.

3.3 Analysis methods

Our analysis methodology employs two fundamental quantities, the calculation of which are described in this section and

summarized in Table 1.

3.3.1 Time-resolved scattering cross-section C(t)205

To quantify tar evaporation in the absence of incandescence, we calculated the time-resolved partial scattering cross-section

C(t) of all particles in the test data sets. This procedure has been described in detail by Laborde et al. (2012b) and is illustrated

in Figure 1. Briefly, the SP2 split detector is used to define the absolute position of particles in the SP2 laser. The effective laser

beam shape is then determined as the median of all measured non-incandescing particles above a user-selected noise threshold.

(We inspected the data to ensure that most non-incandescing particles were also non-evaporating.) Calibrated scattering signals210

are then normalized to this idealized beam shape to obtain C(t).

Above the noise threshold, C(t) is a constant for non-evaporating particles. For evaporating particles, C(t) decreases due to

the decreasing particle volume and, potentially, changing refractive index of the particle (Moteki and Kondo, 2008; Laborde

et al., 2012b). We therefore used the change in C(t) as a method to identify evaporation and quantify the number fraction

of evaporating particles. We calculate the ratio of C(t) at two selected times relative to the mode intensity of the laser beam,215

defined by R(t1, t2) with the times given in terms of laser beam intensity:

R(−20%,20%) =C(−20%)C(20%)

(1)

where C(−20%) represents C(t) at the time when an incoming particle reaches 20% of the laser-beam maximum for the

first time. Conversely, C(+20%) represents C(t) at the time when a particle reaches 20% of the laser-beam maximum for the

second time and has almost left the laser beam.220

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We note that the PSI SP2 Toolkit has long used C(t) for so-called “leading-edge only” (LEO) analysis (Gao et al., 2007). The

calculation of C(t) described above and by Laborde et al. (2012b) is equivalent to a LEO fit of an empirical beam shape function

(rather than a prescribed function such as a Gaussian, and without the constraint of fitting to the leading edge). The results

presented below are therefore an extension of a previously validated and published approach. We also note that our method

for to detecting evaporating tar particles bears some similarity to the method of Moteki et al. (2014) for detecting attached BC225

particles.

3.3.2 Scattering cross-section at incandescence, C(to)

To investigate C(t) after the evaporation of volatile material, we define C(t) at the time of incandescence as C(to). As was also

the case in earlier versions of the PSI SP2 Toolkit, C(to) was defined as the scattering signal which occurred just prior to the

onset of incandescence. The condition “just prior to” is necessary because the filter used in front of the SP2 scattering detector230

transmits a portion of the incandescence signal, and because rBC particles may swell during heating (Bambha and Michelsen,

2015). The to was defined as 2.4µs seconds before ti, the time of maximum incandescence signal. This is illustrated in Figure 1.

We note that earlier versions of the PSI SP2 Toolkit also retrieved C(to) and used it to constrain the apparent refractive index

of rBC, mrBC = (n,k). Because the precise value of mrBC is not well constrained and may vary between BC materials, a range

of possible values for mrBC have been reported in the literature (Bond and Bergstrom, 2006). These values have been empirically235

observed to follow the approximate relationship k ≈ (n−1), as introduced by Bond and Bergstrom (2006) and discussed further

by Moteki et. al. (2010). This empirical relationship is sufficient to constrain mrBC to a single value, within the scope of the Mie

approximation used herein. This method was used in Corbin et al. (2018b) to determine a best-fit mrBC of (1.9, 0.8) for this data

set, which is similar to the value used by Laborde et al. (2012c) for propane-flame soot but smaller than the value of (2.26, 1.26)

often used in SP2 data analysis (Moteki et al., 2010; Taylor et al., 2015).240

3.4 Data filtering

The particle-detection events analyzed herein were filtered to remove events with fitted peak heights below a limit of detection

(LOD) established by inspecting the smoothness of the measured mass distributions. A higher LOD was applied to events

analyzed for C(t). This higher LOD was defined at C(−3%), which is a standard reference condition for LEO analysis in

the PSI SP2 Toolkit and reflects the earliest time at which the signal-to-noise ratio of C(t) becomes acceptable. The LOD at245

C(−3%) was determined by inspecting a scatterplot of non-incandescing particle sizes retrieved at C(−3%) with sizes retrieved

at C(100%), the standard position.

Additional filters were introduced to remove events triggered by noise, events where peak fitting failed (for example, due to

the coincidence of two particles within the laser beam and with one particle touching the edge of the acquisition window), and

events where the detectors were saturated. The number fraction of particles removed by these filters was negligible. For C(t)250

data, only particles with a valid split position could be used, which corresponds to a limit of approximately 160nm in optical

diameter (assuming m= (1.5,0)).

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4 Results

In the following discussion we present results which provided useful evidence for the presence of unique signals from tar

particles in the SP2. Appendix A describes some results which did not provide useful evidence.255

4.1 Evaporating, non-incandescing particles

Figure 2 shows frequency distributions for R(−20%,20%) (Equation 1) for particles where no incandescence signal was

observed. The figure shows data only for particles with optical diameter of 220 ± 40 nm (assuming m= (1.5,0)) to remove

noise from smaller particles and to simplify interpretation.

The R(−20%,20%) represents the fraction by which the scattering cross-section C(t) of a particle decreased when exiting260

the beam (sampled at 20% of the maximum laser fluence) compared to entering the beam (also sampled at 20%). Its value is

close to 1 for a non-absorbing particle and � 1 for an evaporating particle. Values greater than 1 occur due to random error in

C(t).

For both the soot (control) and tar cases, the vast majority of signals can be described by a gaussian fit as falling within the

range 0.7<R(−20%,20%)< 1.3. For the tar case the fitted mean and standard deviation were 1.001±0.001 and 0.279±0.002265

respectively. This standard deviation reflects a 27.9% precision in the retrieval of R(−20%,20%). For the tar case, a substantial

but small fraction of the particles in Figure 2 (578 of 14766 or 3.9%) showed R(−20%,20%)< 0.5, indicating substantial

evaporation. The C(t) profiles of all of these particles are shown in Figure 3. Note that the majority of particles in this sample

were non-absorbing lubrication-oil particles in the sample, which cannot be quantified separately from non-evaporating tar.

Therefore, the actual number fraction of tar particles which evaporate (relative to tar particles which do not evaporate nor270

incandesce) is likely to be significantly higher than the 3.9% given above.

By random inspection of the evaporating particles with R(−20%,20%)< 0.5, we selected a representative example and

plotted its time-resolved scattering signals in Figure 1d. The scattering cross-section C(t) of this example particle begins at

a plateau (indicating unchanging particle size and composition) before decreasing to a second plateau (indicating a second

stable configuration), then ultimately evaporating completely. Not all evaporating and non-incandescing particles showed this275

secondary plateau, many showed only a continuous evaporation, some showed a partial evaporation and remained at a plateau

(Figure 3). It may be speculated that these plateaus reflect the breakup of tar particles into more- and less-absorbing parts,

with the more-absorbing part being completely evaporated and the less-absorbing part passing through the laser unchanged, in

analogy to the particle breakup that is observed for heavily-coated rBC (Sedlacek et al., 2012; Dahlkötter et al., 2014; Moteki

et al., 2014).280

This evaporating, non-incandescing behaviour is unlike any we have previously observed in the SP2. Typically, non-

incandescing particles will show a scattering profile similar to that of Figure 1a, corresponding to constant C(t). Recall that

the beam profiles shown in Figure 1 reflects the median of all non-incandescing particles, and therefore may be interpreted as

illustrating the scattering profile of a typical non-incandescing particle.

9


Evaporating but non-incandescing particles may also be observed in the SP2 when sub-detection-limit rBC is internally-mixed285

with volatile material. This may result in core-shell particles, which were not observed in the tar-rich nor in the control data set.

Alternatively, this may result in the “attachment” of soot particles to non-absorbing droplets, rather than core-shell morphologies.

However, again, this should have been observed in our control data set but was not. Finally, the presence of sub-detection-limit

rBC would have occurred simultaneously with the presence of larger rBC particles, so that some partially-evaporating and

incandescing particles should have been seen (as observed for example by Moteki et al. 2014) if this possibility were significant.290

No particle breakup was obseved in our data; no incandescing particles were observed which had a detectable signal remaining

at C(20%). Therefore, this potential cross-sensitivity can be excluded as affecting our analysis.

4.2 Evaporating and incandescing particles

We investigated the possibility that certain tar particles may incandesce in the SP2 due to laser-induced annealing. This possibility

requires distinguishing incandescing tar from incandescing soot BC particles (and potentially internally-mixed tar–soot particles).295

This distinction could be made using a comparison of the scattering and incandescence signals for tar particles, via the ratio

C(to)/Ipeak, under the hypotheses that tar particles either (i) contain a substantial volume of refractory, non-incandescent

material at the time of incandescence ti or (ii) possess a substantially different refractive index at ti. Our data do not rule out the

possibility that both hypotheses are true. These symbols were defined in Table 1. For this calculation, it is important to use

C(t) and not S(t), because thickly-coated rBC particles penetrate deeper into the SP2 laser. This results in a higher S(t) for the300

same C(t), since deeper penetration into the laser corresponds to a greater photon flux incident on the particle. We note that

the incandescing material in laser-annealed tar would have a significantly different morphology and possibly also molecular

structure than soot BC, which would affect its incandescent properties (Moteki and Kondo, 2010). .

Figure 4 shows C(to)/Ipeak for the tar and soot (control) cases, plotted as a joint-probability histogram of initial optical

diameter. The initial optical diameters correspond to the diameter retrieved from C(−3%) assuming m= (1.5,0) and serves305

to indicate the approximate particle size prior to any evaporation. For soot BC, when the ratio C(to)/Ipeak is appropriately

calibrated, it represents the ratio of optical diameter at ti to rBC-equivalent diameter at ti. The ratio would then represent the

slope of a plot of the rBC optical diameter (just prior to incandescence) against rBC mass-equivalent diameter (obtained from

the incandescence signal), and in this scenario the ratio is constrained as equal to unity during BC-coating-thickness analyses

when the appropriate calibrations are applied (Corbin et al., 2018b). For clarity we have therefore applied these calibrations to310

the data presented in Figure 4, although they do not apply to tar.

Figure 4a shows the C(to)/Ipeak versus diameter histogram for the soot case. An approximately constant C(to)/Ipeak is

observed as a function of initial optical diameter, as highlighted by the dashed ellipse. Above 200 nm, C(to)/Ipeak begins

to decrease, reaching a value of 0.9, which at least partially reflects the limitations of the Mie model used to calculate the

optical diameter in our analysis. The region within this dashed ellipse reflects the SP2 response to soot BC. Inspection of the315

individual particles within the soot BC region showed the canonical SP2 response, as depicted for a representative particle

in Figure 1b. This response involves a scattering signal S(t) that decreases almost simultaneously with incandescence and a

scattering cross-section C(t) that drops rapidly after the onset of incandescence.

10


Figure 4b shows the C(to)/Ipeak versus diameter histogram for the tar case. Here, in addition to the soot BC region, a second

“cloud” of particles appears at higher C(to)/Ipeak and higher initial diameter, as highlighted by the solid circle on the figure. The320

circle is reproduced on Figure 4a to allow a direct comparison of the two data sets. Inspection of the individual particles within

this “tar region” showed an anomalous SP2 response, as depicted for a representative particle in Figure 1e. Unlike the coated

soot particle, no plateau in C(t) is observed at incandescence, rather, incandescence occurs simultaneously with a continuously

decreasing C(t). After incandescence, C(t) is zero. This behaviour indicates that a substantial amount of refractory material

was internally mixed with the material which incandesced. The material which incandesced may have undergone chemical325

transformation during laser heating (laser-induced annealing as discussed above, Sedlacek et al. 2018) or may have been present

prior to laser heating; we consider annealing more likely due to the homogeneous appearance of these tar particles in the electron

microscope (as discussed in Corbin et al., 2019).

The area of Figure 4 containing incandescing tar particles is significantly greater than the area containing soot-BC particles

(that is, the illustrative circle is larger). This indicates that tar particles showed a more variable ratio of non-incandescing to330

incandescing material, and/or that the incandescing material varied in degree of graphitization or annealing. This is the expected

behaviour, considering that the annealing process involves the localized crystallization of graphitic domains following thermal

decomposition, or heating-related internal tensions (Franklin, 1951). Such a phase transition would occur at variable times

during laser heating, leading to a variable ratio between evaporated and annealed material, leading to a variable C(to)/Ipeak

ratio.335

The clear relationship between C(to)/Ipeak and initial optical diameter seen in Figure 4b rules out the hypothesis that this

region reflects extremely-thickly-coated soot. Extremely-thick coatings would show no correlation in Figure 4, and would in

some cases display particle breakup, which results in in a non-zero S(t) and C(t) after evaporation (Moteki and Kondo, 2007;

Sedlacek et al., 2012; Dahlkötter et al., 2014). This was never observed in our data set.

Finally, we also present a coated soot-BC particle in Figure 1c in order to illustrate the behaviour of such particles in the340

SP2. We emphasize that, unlike all other examples, this particle type was very rare in our data set, and is not representative

of our data set, in which most BC was uncoated (Corbin et al., 2018b). This coated particle shows a C(t) that decreases to

a plateau prior to the onset of incandescence. This is the typical behaviour of coated rBC. The initial decrease indicates the

evaporation of a volatile coating, and the plateau indicates continued heating of the now-uncoated rBC up until the onset of

incandescence at ∼ 3500K. The onset of incandescence corresponds to a slight increase in C(t), which must be interpreted345

in the context of heat-induced swelling and interference of the incandescence signal at the scattering detector (Bambha and

Michelsen, 2015). Overall, the scattering and incandescence profile of this coated soot BC particle is clearly distinct from the

profiles of tar. We note that some particles with extremely-thick rBC coatings or coagulated rBC-droplet morphologies may not

display the above-mentioned plateau due to breakup during evaporation (Sedlacek et al., 2012; Moteki et al., 2014), but such

particles also display a C(t) significantly greater than zero after evaporation, and, as mentioned above, were not observed in our350

data set.

11


5 Discussion

The results presented above show that marine-engine tar particles absorb the 1064 nm SP2 laser with sufficient efficiency to

evaporate. In some cases, incandescence accompanied this evaporation, which was attributed to partial laser-induced annealing.

While this incandescence-via-partial-annealing phenomenon was directly demonstrated by Sedlacek III et al. (2018) using355

nigrosin and laboratory-generated tar particles, in our study there is a possibility of rBC pre-existing as an internal mixture with

tar. We consider this possibility extremely unlikely based on the unique formation mechanism of tar compared to soot BC, and

the fact that tar particles have universally been observed as internally homogeneous in the literature (Section 2).

A competing hypothesis to the partial annealing hypothesis is that the incandescing tar particles were actually coagulated

tar–soot-BC particles. We reject this hypothesis, because of the observed late incandescence (in terms of time spent in the laser360

beam) of tar particles. Coagulation would not result in late incandescence; at worst, it would lead to earlier incandescence due to

reduced conductive cooling.

If tar particles absorb 1064 nm light, then they should be measurable by other techniques which employ 1064 nm lasers, such

as the Soot-Particle Aerosol Mass Spectrometer (SP-AMS) (Onasch et al., 2012) and pulsed laser-induced incandescence (pulsed

LII) instruments (Michelsen et al., 2015). Corbin et al. (2019) explored the response of the SP-AMS to the same tar particles365

discussed in the present manuscript and found no substantial difference between the mass spectra of tar-containing and tar-free

samples. This lack of difference may be due to the different conditions that particles experience within the SP-AMS (including

lower pressure, shorter beam exposure times, and different laser power densities, as mentioned in the introduction), or due to the

fact that the SP-AMS used in our study did not obtain the single-particle measurements that allowed the SP2 to differentiate

between tar and lubrication-oil-related particles. Future work should explore the response of pulsed LII instruments to tar.370

The number fraction of evaporating tar particles observed in our data set was 3.9% at 220 ± 40 nm. This fraction is biased by

the fact that the majority of particles in our sample were lubrication-oil related (Corbin et al., 2018b). The number fraction of

incandescing tar particles was 1324 of 2.62× 105, or 5.1%. For a given optical diameter, tar particles generated much smaller

incandescence signals than soot particles (Figure 4), so that the actual bias in SP2-determined rBC mass concentrations due

to tar incandescence was � 5.1%. Considering that the mass of tar brC was threefold greater than the mass of soot BC in our375

measurements (Corbin et al., 2019), this bias is negligible relative to the typical 15% accuracy of an SP2 mass calibration

(Laborde et al., 2012c; Taylor et al., 2015).

It cannot be overemphasized that the material referred to as tar or “tar balls” is partially-graphitized, amorphous carbon. There

is no therefore no well-defined molecular structure for tar, and a given tar sample may lie at some point along a continuum of

graphitization (Bond, 2001; Corbin et al., 2019). While the same is also true of soot BC (Minutolo et al., 1996; Vander Wal380

et al., 2014), the literature indicates that the range in degree of graphitization of BC emitted by common combustion sources

(Bond and Bergstrom, 2006; Zangmeister et al., 2018) is narrower than the corresponding range for tar (Corbin et al., 2019).

Care must therefore be taken when extrapolating the present results to other studies. Certain tar samples may be less graphitic

(and less likely to undergo laser-induced evaporation) or more graphitic (and more likely to undergo laser-induced incandescence)

than our samples. Based on the wavelength-dependence of absorption of tar in our samples reported and placed in the context of385

12


literature by Corbin et al. (2019) (AAE of about 3), we believe that our tar samples were of a typical degree of graphitization.

Future studies should explore the possibility of modulating the SP2 laser power (Sedlacek et al., 2012, 2015), to provide

additional information by which tar and soot BC may be distinguished.

6 Conclusions

We investigated the response of the SP2 to near-infrared-absorbing, refractory (to about 1000K) carbonaceous particles (“tar”)390

using a data set in which the presence of tar has been demonstrated, and a control data set in which such particles were absent

(Corbin et al., 2019).

By inspecting the time-resolved scattering cross-sections C(t), we found that tar particles can be observed as evaporating

but non-incandescing particles in the SP2. Some tar particles also incandesced, either due to laser annealing or possibly due to

chemical heterogeneity of the material being referred to as tar. These incandescent tar particles were clearly distinguishable395

from soot BC according to the ratio of scattering-at-incandescence to incandescence signals. This ratio was a factor of 1.2 to 2.0

greater for tar than for soot BC and much more variable. This high degree of variability would be expected if the incandescent

material in these particles formed via the localized crystallization of graphitic domains during laser-induced annealing, such that

the molecular composition of the incandescing tar particles may vary significantly.

In our data set, we identified 578 and 1324 particles as non-incandescing or incandescing tar particles, respectively. Assuming400

that the probability of false-negatives is similar for these two cases (in other words, assuming that our different methodologies

were not more sensitive to either incandescing or non-incandescing tar), this indicates that about 70% of tar particles produced

incandescence signals in the SP2. Future work should employ laboratory-generated tar balls or morphology-based classification

to more accurately estimate what fraction of tar particles can be expected to evaporate in the SP2 laser.

The analysis presented here shows that an SP2 equipped with a split detector is capable of detecting tar. This makes the SP2,405

to our knowledge, the only high-throughput technique which has the potential capability of distinguishing tar from soot particles

or soluble brC. It remains undetermined whether or not the SP2 signals are useful for the quantification of tar mass or number

fractions. Based on the fact that our tar particles had optical properties similar to those reported in other studies (discussed in

Corbin et al., 2019), we estimate that our tar particles are of a typical degree of graphitization, such that other tar-containing

samples should display similar behaviour to that observed herein. Future work should also explore the possibility of modulating410

the SP2 laser fluence in order to exploit differences in the absorption efficiencies of tar and soot, while keeping in mind that the

material referred to as tar lies on a continuum between amorphous carbon and highly graphitic carbon, such that certain samples

will absorb 1064 nm light more effectively than others, while also most likely being more refractory (Corbin et al., 2019). The

techniques used herein may be useful for the future identification of the presence or absence of tar in a sample of unknown

composition.415

13


Author contributions

JCC and MGB conceptualized the study; JCC performed the research and drafted the manuscript with input from MGB; MGB

developed the original SP2 analysis code which JCC further developed; and MGB acquired funding.

Acknowledgements. This work was supported by the ERC grant ERC-CoG-615922-BLACARAT and by Natural Resources Canada under

the Program of Energy Research and Development (PERD). We thank our previous coauthors (Corbin et al., 2018a, 2018b, 2019) for their420

contributions to generating the data sets used here.

14


Table 1. Table of symbols used in the text.

Symbol Meaning

t Time spent by a particle in the SP2 laser beam

S(t) SP2 scattering signal

C(t) Scattering cross-section corresponding to S(t)

C(−3%) Scattering cross-section at -3% of laser maximum

as a particle enters the SP2 laser beam

I(t) SP2 incandescence signal

Ipeak SP2 incandescence signal at peak

to Time just before onset of incandescence

ti Time of maximum incandescence

R(−20%,20%) Ratio of C(t) at two different t (Eq. 1)

Appendix A: Diagnostics which did not differentiate between tar and the control data set

We attempted to identify evaporating particles using a number of different statistics, with the goal of identifying a parameter

which was sensitive to evaporation without requiring the split detector. One motivation for a split-detector-free method is that

the new model of the SP2, SP2-XR, does not contain a split detector. Using the region of the scattering signal identified as a425

peak by the PSI SP2 toolkit, we calculated the full-width-half-maximum (FWHM), the ratio of FWHM to full width, the peak

skewness, the mean absolute difference (MAD) between either half of the peak, and the Kolmogorov-Smirnov statistic. These

statistics were evaluated by manual inspection and by comparison to R(−20%,20%).

Manual inspection of the MAD suggested that this statistic successfully isolated tar particles, however, in terms of probability

density functions, the differences were not different from the control case. Manual inspection of the FWHM of the incandescence430

peak also suggested a difference for the tar case, but further analysis showed that this difference was due to tar particles

penetrating deeper into the laser beam, and therefore experiencing higher heating rates at incandescence. We also explored

the use of the Moteki and Kondo (2008) approach to identify evaporating, non-incandescing particles, but did not identify

conditions where this method was successful. Based on plots similar to Figure 3 but for various subsets of particles, we believe

that alternative approaches must be explored, such as the machine learning approach introduced by Lamb (2019). Alternatively,435

future work may be able to distinguish tar-containing particles without a split detector if additional instrumental parameters,

such as the laser fluence, are varied.

15


Figure 1. Particle types observed in this study. These examples were selected to be representative of the trends shown in subsequent figures. a)

Typical non-absorbing and therefore non-evaporating particle. (Likely lubrication oil / sulfate mixture.) The labels −20% and +20% indicate

fractions of maximum beam intensity. b) Typical soot BC particle. c) Atypical coated soot BC particle (rare in this data set, selected for

illustration only). d) Typical evaporating but non-incandescing tar particle. e) Typical evaporating and incandescing tar particle. Note the

difference between position of maximum incandescence (vertical blue lines) and position of stable C(t) in panels c) and e), as highlighted by

the vertical blue lines. Note also that the ordinate scales vary in order to highlight key features.

16


Figure 2. Frequency distribution of the ratio of scattering cross-sections, R(−20%,20%) for non-incandescing particles. curves are Gaussian

fits. Ratios were calculated by dividing the second scattering cross-section measurement at 20% of maximum beam fluence (particle exiting

the SP2 laser) by the first (particle entering the SP2 laser). Random error in R(−20%,20%) was modelled by gaussian fits, which indicate

that about 3.6% of tar-case particles evaporating to less than half of their original cross-section. Note that this 3.6% reflects the fact that the

majority of particles in this sample were non-absorbing lubrication-oil particles. In the soot test case, no non-incandescing particles evaporated.

Figure 3. C(t) for all particles with R(−20%,20%) < 0.5 in Figure 2, normalized to C(−3%) (which is normally using for coating-thickness

analysis). Each transparent red line represents C(t) for a single particle, as was also shown in the lowest panel of Figure 1d.

17


Figure 4. Incandescing tar particles distinguished from soot BC by their scattering properties in the SP2. Scattering cross-section just prior

to incandescence (C(ti); see Figure 1 for illustration) normalized to maximum incandescence signal I(ti), plotted as a function of particle

optical diameter prior to evaporation. Under the condition that the incandescing material is comparable in all cases, an increase in C(ti)/I(ti)

indicates the presence of non-BC refractory material. Inspection of the data indicated that the joint probability is not exactly equal to zero in

the soot-BC case due to coincidence (simultaneous presence of two particles in the laser beam).

18


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detection of tar brown carbon with the single particle ... · the sp2 is normally used to report...

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